How to Check if Your Car's Thermostat is Working

by Contributor

We've all been there before. The car is overheating, but you do not know what the problem is. While many people dive in, assuming the worse and replace every part under the hood, one small inexpensive part is often to blame, particularly in older cars. Liquid-cooled engines have a thermostat that keeps coolant out of the engine while it heats up and allows coolant in once it gets hot. If the thermostat is not working, or failed shut, coolant will not flow freely into the engine and the engine will heat up and eventually seize. By simulating the engine/coolant heating up outside the engine you can tell if the thermostat is to blame for your problems.

Checking if Your Car's Thermostat is Working

Pop the hood and locate the thermostat of your vehicle. The thermostat is generally located inside a hose between the radiator and the engine. It is round and about two inches in diameter.

Using the socket set, unbolt the thermostat and remove it from the vehicle.

Place the thermostat in a pot of water and begin to heat it. Most thermostats are set to operate at just below boiling. As the water heats watch the thermostat closely. Just before the water begins to boil a working thermostat will open, as if to allow coolant into the engine. A jammed or failed thermostat will not open.

Replace the part if the thermostat does not open. If it does open, place it back in your car--you have a more serious problem and should take it to a mechanic.


  • check If the thermostat is failed and you replaced it, that may not be the engine's only problem. Keep and eye on your gauges. Never drive a car if the temperature is higher than its normal operation.


  • close The coolant in an overheated car can get very hot--proceed with caution.
  • close Allow the engine to cool before removing the thermostat from the vehicle.

Items you will need

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